Windows 7 - What is best free anti-virus software?

A young friend has asked me if there is a reliable anti-virus software that is available free of charge. I searched the subject on Google, but the responses were not clear (to me). I will appreciate anyone here sharing their experience and ideas about this. Thanks.

These threads will give you an idea on how many times this has been asked.
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THW

A popular combination is microsoft Security Essentials and Malwarebytes Antimalware.

You're going to get a lot of different responses as the subject is up for debate and everyone has their own preferences. Start by looking at this post and read the pros & cons about each AV

What's the Best Anti-virus?

The best AV is the one you feel comfortable with, and keep updated on a regular basis. Also, one of the biggest factors is common sense & staying knowledgeable about threats on the web.

There are also helpers you can use, on demand AV scanners such as Malwarebytes, Microsoft Safety Scanner & Malicious Software Removal Tool. Running these on a regular basis is a good idea as not all AV's are 100% effective.

And, a firewall is a must have item nowadays

Fixed

There's no best free anti-virus but if i were to choose, it would be Avast. It's the most complete free anti-virus you can get. You'll see others in here that will recommend Microsoft Security Essentials but personally i would not use it. MSE does not have any self-defense module which can easily be disable or turn off by a malware that is infecting a PC.

I have used most of the free AV(MSE, Avira, Panda Cloud Free, Avast) and always end up going back to Avast. I feel safer with it and it hasn't let me down.

Keep an eye out on newegg.com and frys for full rebates on security software.

THAT's what I meant to say....

As others have said, no antimalware product will be 100% effective 100% of the time. If there was such a creature we'd all be using it!

Some other considerations include:
1. What are the specifications for the specific computer (like CPU, RAM, etc.) Some antimalware programs are more "bloated" than others and could slow a machine down.
2. What programs (software) are installed and will they play nice with the chosen antimalware product?
3. What are the individual requirements of the user(s)? Will the machine be used to download torrents? Visit questionable sites? In other words, even a top rated antimalware program could be pushed beyond its limits.

Since we're talking about free products, there's nothing better than to give them a try. Even the paid products usually have a free trial period. Put each product to the test based on your friend's real life, day-to-day activities.

CAUTION: Make sure your friend creates a system image (not just a restore point.) That way, when the computer becomes infected he/she will be able to revert back to a clean image without having to manually reinstall everything on the hard drive.

Thank you all! My young friend and I are together reading ... and following ... all the responses. They are much appreciated. This is a great forum.